The broadcast nature of wireless communication systems makes them vulnerable to wiretapping by a malicious eavesdropper. Physical-layer security (PHY-layer security) provides a cost-effective solution to this problem through secret communications that do not require a hand-shaking mechanism between the communicating parties [1a], [2a]. Several approaches in the area of PHY-layer security have been proposed in the last decade (see [3a] and reference therein). Among the most promising is the method of artificial noise (AN), also known as friendly jamming (FJ), which guarantees a nonzero secrecy rate for a user without knowing the eavesdropper's location [4a]. Secrecy rate (capacity) is defined as the maximum rate at which a transmitter (Alice) can securely transmit information to its legitimate receiver (Bob). This rate is the difference between the mutual information between Alice and Bob, to that between Alice and an eavesdropper (Eve). Basically, in the AN method, Alice uses multiple antennas to generate AN, increasing the interference at Eve but not interfering with Bob.
The increasing demands for wireless services together with the scarcity of wireless spectrum forces users to share the same band. Sharing the spectrum leads to interference among users. To accommodate simultaneous transmission of several information signals in a network, the friendly jamming signal of each transmitter must be designed to not interfere with other legitimate receivers in the network. To avoid such interference and yet prevent the leakage of the transmission signature, one solution is to exploit multiple input, multiple output (MIMO) precoding to ensure that the null space of any friendly jamming signal includes the locations of all legitimate receivers but excludes potential eavesdropping locations. This solution, however, is not practical in situations where coordination between legitimate transmitters is challenging (e.g., mobile ad-hoc networks). Therefore, the need for interference management is crucial to guarantee secure yet non-interfering communications. Interference management roots back to the power control problem in traditional interference channel networks, which has been extensively investigated ([5a], [6a]). The main challenge there is to manage the interference at all of the receivers so as to maximize the sum of individual rates.
In a similar way, in the interference wiretap channel, the unwanted interference from one transmitter degrades the received signal power at other receivers, reducing the efficiency of the network in terms of secrecy rate. However, the possibility of increasing interference at Eve makes the unwanted interference potentially useful in assisting a more secure communication. This idea was investigated in [7a], [8a]. The effect of interference alignment in providing secure transmissions was investigated in [9a] and [10a]. Other approaches employ special codebooks for the transmitters to improve the secrecy in the network [11a]. In some studies, a particular user is assumed to be eavesdropped on, and the objective is to maximize the secrecy rate of that user while satisfying a rate constraint for other users in the network. In other words, users coordinate with each other to increase interference at the eavesdropper while maintaining their own rate requirements. As an example, in [12a] the authors considered a two-user single input, single output (SISO) interference channel with an eavesdropper. By jointly optimizing the transmission powers of the two users (without AN), the authors tried to maximize the secrecy rate for one user while maintaining a given quality of service (QoS) for the other user. Another example is [13a], where the authors study this problem in a two-tier heterogeneous downlink comprised of one macrocell and several femtocells. In particular, the authors propose a transmit beamforming method for the signals intended to both macrocell users and femtocell users so as to maximize the secrecy rate of one eavesdropped macrocell user. This maximization is subject to satisfying the rate requirements of all other macrocell users.
In other works, PHY-layer security was studied when users have confidential messages and there is no eavesdropper in the network. In other words, the transmission of one user is not to be captured by unintended receivers. In [14a] a MIMO interference channel with confidential messages was studied using game theory to find an operating point that balances the network performance and fairness. The work in [15a] considered the secrecy rate region of the interference channel when users transmit AN along with their data. They showed that by using AN, the secrecy rate region will be larger than when AN is not employed.
As wireless mobile systems continue to be widely adopted, confidentiality of their communication becomes one of the main concerns due to the broadcast nature of the wireless medium. Cryptographic techniques can be utilized to address these concerns, but such techniques often rely on computational limitations at the adversaries (an assumption that may not hold with advances in computing power) and are mostly based on unproven conjectures (e.g., hardness of some computing problems). Physical (PHY) layer security, on the other hand, can be implemented regardless of the adversary's computational power. It also takes advantage of the characteristics of the wireless medium.
Wyner [1 b] initiated the concept of secrecy capacity by defining the degraded wiretap channel. The authors in [2b] extended Wyner's work to non-degraded discrete memoryless broadcast channels. Later on, the secrecy capacity of MIMO wiretap channel was determined [3b]. In [4b], the authors obtained the secrecy region of the Gaussian MIMO broadcast channel. To guarantee secrecy, Goel and Negi [5b] introduced the concept of artificial noise, a.k.a. friendly jamming. The idea is to artificially generate noise over the channel in order to degrade eavesdropping. The authors in [6b] used a similar approach for security in MIMO wiretap channels under imperfect channel state information (CSI). Cooperative jamming strategies for two-hop MIMO relay networks in the presence of an eavesdropper that can wiretap both channels were proposed in [7b]. The authors in [8b] studied a multiuser broadcast channel where a sender transmits K independent streams to K receivers. They jointly designed linear precoding of the transmitted signals and cooperative jamming so as to enhance PHY security. A full-duplex (FD) receiver that sends artificial noise for secure communication was proposed in [9b]. At least two antennas are needed at the receiver, one for sending the jamming signal and the other to receive the information message. A similar system model was used in [10b]. However, the authors in [10b] did not assume complete self-interference suppression (SIS) as in [9b].
Another system model with one full-duplex base station (BS), one transmitter, one receiver, and one eavesdropper was considered in [11b]. In this model, the BS receives a message from the transmitter while sending an information message to the receiver together with a friendly jamming (FJ) signal. The authors assumed that the transmitter's signal does not interfere at the receiver, and they solved the problem of maximizing the secret transmit rate. Remarkably, none of above works includes receivers with “full-duplex antennas”, as multi-antenna FD receivers considered therein refer to having some antennas exclusively used for receiving data and others to send FJ signals. Moreover, none of these studies consider a multiuser scenario where receivers send friendly jamming signals. In contrast, the present invention considers a K-user scenario (where K is greater than or equal to two) with single-antenna full-duplex receivers that also generate their own friendly jamming signals. Receivers are not allowed to decode any transmitted information signal not intended for them (confidential communications), and the information leakage to eavesdroppers for each information message is vanishing. In addition, the present invention also addresses scenarios where eavesdropping channels are correlated with those of legitimate receivers.
The broadcast nature of the wireless medium exposes communications to eavesdropping and privacy attacks. Although cryptography can be used to protect the information secrecy of a data frame's payload, it is not sufficient to prevent the leakage of side-channel information from unencrypted headers. Moreover, in many wireless standards, such as 802.11, management and control frames are often sent in the clear. Various operations of a wireless protocol, such as establishing session keys, rely on the exchange of these frames. Information theoretic secrecy [1c], [2c] at the physical (PHY) layer is a lightweight approach that aims at preventing an eavesdropper (Eve) from decoding a plaintext frame. A transmitter (Alice) and its legitimate receiver (Bob) are guaranteed secret communications if the Alice-Bob channel is better than Alice-Eve channel. In [1c], the notion of secrecy capacity was introduced as the maximum rate at which Alice can securely transmit information to Bob. This rate is the difference between the mutual information between Alice and Bob, to that between Alice and Eve.
Non-zero secrecy capacity is not always possible. For example, if Eve is closer to Alice than Bob, then the Alice-Eve channel may be better than the Alice-Bob channel, resulting in zero secrecy capacity [1c]. Friendly jamming (FJ), proposed in the pioneering work of Goel and Negi [5b], can be used to degrade the Alice-Eve channel without harming Bob's reception. Essentially, a FJ signal is a randomly generated artificial noise. To nullify the FJ signal at Bob, the authors in [5b] considered the case when Alice has multiple antennas. Alternatively, a bank of relay nodes can be utilized to transmit the artificial noise in the null space of the Alice-Bob channel.
Although FJ-based PHY-layer security has been extensively considered for single-link scenarios, only a few papers studied the problem in multi-link scenarios. Research efforts on secret communications in a multi-link network can be classified into two broad categories: Large-scale [4c], [5c], and [13c] and small-scale wireless networks [6c], [7c]. Considering a large-scale wireless network consisting of n nodes, the authors in [4c] derived the per-node asymptotic secrecy capacity. They also proposed to use “Rx-based FJ”, whereby legitimate full-duplex receivers are able to cancel the self-interference resulting from their generation of FJ signals. For the case of independent eavesdroppers, it was shown that a per-node secrecy capacity of θ(1/√{square root over (n)}) is achievable, which is the same per-node capacity without secrecy considerations. These results imply that the per-node secrecy capacity is not affected by the presence of eavesdroppers. However, placing the FJ devices at the same locations of the communicating nodes may not be optimal from an energy consumption perspective. The interference of Rx-based FJ on other receivers was also not considered in [4c]. The authors in [5c] explored allowing a fraction of transmitters to cooperatively send their signals to their receivers through relay nodes, i.e., two-hop communications. The idea is based on the work in [8c]; wherein, for each Alice-Bob pair, a relay node with “good” channels to Alice and Bob is selected. Relay nodes with “bad” channels to the selected relay or to Bob are used to produce FJ signals to confuse passive eavesdroppers. Instead of generating FJ signals, simultaneous transmissions are exploited in [5c] to create high interference at the eavesdroppers. In this sense, the messages of other Alice-Bob pairs are utilized as FJ signals. Secrecy is guaranteed only as n tends to infinity. The results of large-scale wireless network, however, may not be always applicable to small-scale networks that can have irregular topologies.
Secure minimum-energy routing with the aid of FJ was investigated in [6c], [7c] for a small-scale network. The objective is to compute a minimum-energy path subject to constraints on the end-to-end communication secrecy and the throughput over the path. The authors proposed a power allocation scheme to assign FJ power required to secure individual links. The secure routing problem was reduced to finding a path with the minimum total information and FJ power. Each link was studied independently, assuming that it can be secured by its own set of FJ devices, and there is a discrete set of eavesdropping locations, each with a given probability of eavesdropping in that location. These works, although applicable to small-scale networks, do not jointly consider the optimal placement and power allocation of the FJ devices. Moreover, they do not exploit the FJ devices associated with a given link to help in providing secrecy for another link, which can reduce the total jamming power. Finally, they assume that the FJ signals are nullified at legitimate receivers, but the conditions needed to ensure such nullification are not incorporated in the formulation. This undermines the applicability of their designs.
Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.